Passive radar emitter direction-finding (DF) utilizing radio frequency (RF) interferometers mounted on aircraft requires finding the emitter's azimuth and elevation in the observer's local-level reference frame. In the description given hereinafter the term aircraft is meant to encompass any observational platform whose motion involves attitudinal changes, such as roll, pitch and yaw, as well as translational motion. In particular, an interferometer sensor array mounted on the leading edge of an airplane wing measures AOA in relation to the sensor's own system of three dimensional coordinates which are then transformed to the observational body's frame of coordinates and a level frame of coordinates to report azimuth and elevation. Finally target location is reported in a set of coordinates for the Earth.
Finding emitter elevation and azimuth from aircraft has previously required the use of planar or conformal interferometer arrays. A linear interferometer could not be used, since a single linear array measures angle-of-arrival (AOA) and not direction-of-arrival (DOA). That is, a single linear interferometer produces an AOA cone, and the emitter can be anywhere on the intersection of that cone with the Earth.
A linear interferometer designed to fit on the leading edge of an aircraft wing, is illustrated in FIGS. 1a and 1b. FIG. 1a illustrates a typical prior art linear interferometer array 10 having a plurality of antennas or sensors 12 arranged as shown to have a baseline d . In FIG. 1b array 10 supplies through SPST switch 14 received radar signals to phase detector/receiver 16. The phase information from the receiver is supplied to a processor 18 which accomplishes phase ambiguity resolution; the phase-resolved signal 15 is then used for determination of AOA information, as shown at 19. The phase measurement of a plane wave with unit normal (DOA vector) u across one baseline d is ##EQU1## Thus the quantity measured is the angle of arrival, or AOA between the interferometer baseline and wavefront. Any emitter lying on the AOA cone will produce the same measured phase .phi..sub.m. For a planar earth approximation, this means that any emitter lying on the hyperbola resulting from the intersection of the AOA cone with the earth can generate the same AOA, and hence emitter azimuth is available from this single measurement only in the special circumstance that the emitter lies in the plane containing the linear array baseline. When this is not true an ad hoc assumption about emitter elevation must be made, typically that the emitter lies on the radar horizon. For emitters not on the horizon the error in using AOA as the true azimuth measurement, typically called the "coning" error, is given by ##EQU2## This equation is strictly only true for the sensor coordinates but this caveat is not important here. Equation 2 indicates the azimuth error becomes quite large when emitters are at steep elevations. It is negligible for emitters on the horizon if the aircraft is flying level, but may become important even for distant emitters when the aircraft has a significant roll or pitch attitude.
Since emitter azimuth is an important parameter in many systems performing passive radar detection, e.g. for emitter classification and identification, this coning error is a severe drawback. Another deficiency AOA-only systems suffer is their lack of elevation measurement prevents "az/el" emitter location, i.e. finding emitter range r using phase measurements made in a single dwell along with the observer altitude h via a relationship such as ##EQU3## However linear interferometer systems do perform location using bearings-only passive location techniques. Bearings-only ranging utilizing AOA essentially finds the intersection of the multiple AOA-hyperbola generated as the aircraft moves along its track. The accuracy of any such AOA-only location technique is characterized by a "geometric" signal-to-noise ratio or gSNR ##EQU4## where .DELTA.B is the bearing spread at the emitter created by the observer's motion, and .sigma..sub.az is equivalent to .sigma..sub.AOA (for the purpose of characterizing bearings-only accuracy) when the aircraft flight is essentially straight and level.
Thus a positive feature of bearings-only ranging is that the range accuracy can be improved by making the bearing spread larger. This is in marked contrast to az/el location. Az/el location estimates cannot be refined by sequential averaging because of the large bias errors typically present in the elevation measurement. These elevation errors can have a significant DOA dependent component, and since an important application of az/el ranging is to locate emitters near the observer flight path, the DOA may not change significantly. DOA dependent bias error is also present on the bearings-only measurement, but has a negligible effect for emitters at bearings essentially normal to the observer's flight path, which is often the case when AOA-only location is used.
FIG. 2 (described below) summarizes the operation of an AOA-only system. This system, as in FIG. 1b, supplies phase information from receiver/phase detector 16 through switch 20 to processor 21 which provides ambiguity resolution for each of the arrays 10. The resolved baseline information is then used at 22 to compute AOA information in accordance with the sensor's set of coordinates. After a range estimate is supplied at 23, the AOA information is calculated in accordance with the platform or body's set of coordinates at 24, and then to level coordinates at 25.
The benefits of using a single linear array doing bearings only ranging are the limited number of phase measurements required per dwell compared to multi-dimensional arrays, and compact installation. The drawbacks are the inability to go from AOA in the sensor frame to azimuth in the level frame without assuming an ad hoc emitter elevation, i.e. introducing an unknown and possibly large coning error, and the lack of any elevation measurement to use in rapidly estimating emitter range, particularly for emitters close to the aircraft.
Overcoming these drawbacks and providing accurate azimuth has previously required utilizing an interferometer array extending in at least two dimensions, such as the conventional conformal array. In the latter array it is necessary to use a vertically disposed sensor array to form an elevation baseline while another sensor array generally disposed horizontally is used to resolve the elevation array ambiguities.
The phase measurements in such a multi-baseline system cannot typically be made without receiver switching between the baselines, i.e. on a nonmonpulse basis. Besides increasing system complexity, such baseline switching complicates the detection of multipath errors on the phase measurements.
There are other problems with switching. In the conformal array discussed above the horizontal array must have its phase measurements completed before making phase measurements on the elevation antennas. If the emitter is no longer detected after the "horizontal" phase measurements are made, because, for example, of emitter scanning or terrain blockage, elevation will not be obtained.
Note that the possibility of not getting a full set of phase measurements is increased by the use of elevation arrays on low RCS (Radar Cross Section) aircraft, since stealth aircraft impose RCS restrictions on the antennas. Adding an elevation array increases the overall RCS, requiring antenna design trade-offs that reduce system sensitivity and hence may prevent detecting emitter side and backlobes.
Obtaining the space to mount a planar array or three dimensional array is difficult on many smaller aircraft. Also, important delta-wing stealth aircraft designs do not provide extensive vertical area, and hence little space for a planar array no matter what the intrinsic aircraft size. Although by utilizing conformal design techniques elevation arrays can be mounted on the leading edge of delta-wing aircraft, the antenna elements do not have common boresights. This can introduce significant bias errors, especially when certain popular ESM system antenna elements, such as broadband multi-arm spirals are used.
A positive aspect of multidimensional arrays is that, aside from the time required for baseline switching during a dwell, az/el location provides near monopulse emitter location. But this very desirable feature is mitigated by the following deficiency:
The 1-.sigma. accuracy of az/el location is characterized by a "geometric" signal-to-noise ratio (gSNR) EQU cot(e).sigma..sub.e ( 5)
Thus the estimate is intrinsically inaccurate at lower altitudes and at any altitude for emitters near the horizon, with no means of subsequent refinement, i.e. no bearing spread factor as for sequential AOA location.
Because of multi-dimensional array installation limitations, baseline switching-induced system complexity, and intrinsic inaccuracy of az/el ranging, linear interferometer arrays measuring AOA-only have been used extensively in ESM systems. Flight tested linear interferometers working over emitter frequencies from 2 GHz to 18 GHz, such as those designed by the Amecom Division of Litton Systems, Inc. for the TEREC system and an advanced capability receiver for the EA6-B program are readily available. But, as noted above such arrays do not provide true emitter azimuth or monopulse location.
It is therefore, an object of this invention to allow such linear arrays to be used to perform the functions associated with more complex multi-dimensional arrays, i.e generate azimuth, elevation, and emitter range in a manner that does not involve flying a base leg to produce bearing spread.
It is the further object of this invention to allow emitter location by multiple platforms collocated, as in a formation, and to remove the defects associated with conventional az/el location by converting systematic bias errors to errors random in time, thus allowing improvement of the az/el range estimate by sequential averaging.